Investment casting is a process for making articles, referred to as castings, from metals and alloys. Castings are formed by pouring molten metals or alloys into molds having an internal cavity shaped in the form of a desired article. The metal or alloy is allowed to solidify inside the mold, and the mold is then separated from the casting. The mold is typically made from refractory materials, such as ceramics.
Investment casting molds can be formed by a dip coating-process in which wax or polymeric “patterns” formed in the shape of a desired article are dipped into a casting slurry. A casting slurry is a system in which small particles, such as small particles of refractory materials, are substantially uniformly dispersed in a liquid. The liquid can be a variety of materials, such as water or an organic material, including but not limited to alcohols. The wet slurry material is allowed to at least partially dry to form a covering over the pattern. The pattern can be repetitively dipped to build up a coating of the desired thickness.
Aqueous slurries for processing ceramics are relatively inexpensive and environmentally safe. However, aqueous processing of some refractory materials, such as ceramics, is difficult because they dissolve in water over time. Organic-based slurries also have this problem because such slurries typically include water. A common pathway for dissolution of ions from ceramics is believed to involve hydration of surface ions. “Hydration” means that the ceramic particles react with water to form a chemical bond. The compounds formed by hydration are referred to as hydrates. Surface ions adsorb water, react with the adsorbed water to form hydrates, and then dissolve.
Aging is defined as any change in a slurry property or properties versus time. Hydration of refractory powders results in slurry aging by, for example, increasing slurry viscosity. Aging can contribute to lower shell quality in many ways. For example, as ceramic materials dissolve, the dissolved species may substantially change the ionic strength of the solution and consequently the particles agglomerate. This can adversely affect mechanical properties of the constructed shell. Furthermore, if the ionic charge of the species that dissolves is different from the ionic charge of the ceramic particles or other components of the slurry (e.g., the ceramic particles have negative charge, but the dissolving species have positive charge or vice versa), then the dissolving species preferentially may adsorb onto different components of the slurry. This may change the theological (the deformation and flow of matter) properties of the slurry, as well as other slurry properties of interest. Finally, as ceramic materials react with water, some ions preferentially may dissolve relative to others, which consequently changes the ratio of ions in suspension and solution. This may result in changes in the physical or chemical properties of the ceramic.
Commercial dip-coating processes require using large volumes of slurries. It may take weeks or months before the entire quantity of a particular slurry is consumed. Unfortunately, slurry aging typically requires that the slurry be discarded well before the entire quantity of the slurry initially formulated can be used to form casting molds. This is both costly and wasteful.
Powders used to make casting slurries are substantially free of surface hydroxides immediately after being manufactured at high temperatures. Slurry materials may be exposed to high temperatures during the initial manufacturing processes and during any subsequent fusion or sintering processes. Fusion and sintering are two methods used to increase the particle size of refractory powders. Fusion involves heating a powder above its melting point to produce a liquid. The liquid is cooled, re-crystallized and ground into a more desirable particle size distribution. Sintering involves fusion of fine particles upon heating at temperatures below the complete melting point of the powder. When sintering is complete, the sintered material is ground to a desirable powder size.
However, once the manufactured powders are exposed to ambient water in the atmosphere, hydration begins. Because refractory powders are bulk manufactured and often transported over long distances, it is not always practical or cost-efficient to either use or adequately seal the powders immediately after they are manufactured. Moreover, even if the powders are sealed at the factory sufficient to prevent hydration, which generally is not the case, a consumer who does not use all of the powder once it is unsealed has to store the unused portion. Unless the powders are (1) used immediately after they are manufactured, or (2) sealed in a water-free container immediately after they are manufactured and subsequently used immediately after unsealing, they will undergo surface hydration. In practice, neither 1 nor 2 are practicable; hence, refractory powders typically used to form casting slurries are hydrates.
Hydration of refractory materials may be temporarily reduced by the consumer if the consumer undertakes further processing of the powders, for example by sintering or fusing the powders, after they are received from the original manufacturer. However, the resulting powders immediately begin to rehydrate unless steps are taken to prevent hydration.
A number of solutions have been offered to control aging of investment casting slurries. Horton's U.S. Pat. No. 4,947,927 shows that increasing the pH to above 11 can reduce aging of yttria slurries. This is because yttria dissolution decreases with increasing pH. However, maintaining yttria slurry pH in a production environment above 11 at all times is inconvenient and impractical. Furthermore, compositions exhibit increased toxicity as the pH varies significantly from neutral.
Yasrebi et al.'s U.S. Pat. Nos. 5,407,001 and 5,643,844 teach decreasing the overall dissolution rate of an oxide by doping the oxide with a material having a lower solubility in the slurry medium, typically water, than the oxide. Consequently, slurry aging can be reduced. Coating powder surfaces with a protective layer also can reduce slurry aging. Yasrebi et al., U.S. Pat. No. 5,624,604, shows that adsorption of hydroxylated benzoic acid onto the surface of rare earth oxides reduces their dissolution rate and thereby reduces slurry aging.
Persons skilled in the art of ceramic processing have long sought simple and inexpensive methods to increase the lifetime of casting slurries. Despite the prior inventions directed to this objective, there still is a need for convenient and practical methods for increasing the useful lifetimes of investment casting slurries.